1. Field Of The Invention
The present invention relates to a long-nip roll press comprising a press shell that passses continuously over a press shoe supported in a recess on a supporting member, and a mating roll that opposes the press shoe.
2. Description Of Related Art
The following publications are in the background of the present invention:
1. U.S. Pat. No. 4,625,376 issued Dec. 2, 1986, commonly assigned herewith, whose disclosures are incorporated by reference herein.
2. U.S. Pat. No. 4,643,802 issued Feb. 17, 1987, commonly assigned herewith, whose disclosures are incorporated by reference herein.
3. German Democratic Republic Patent 79,919 issued Feb. 12, 1971.
4. Federal Republic of Germany Patent 3,030,233.
Long-nip presses are successfully used in press ends of paper machines. Their advantage over ordinary roll presses is that, due to the extended length of the press nip in the direction of travel, better water removal performance can be obtained. By the use of a tubular press shell fastened on supporting disks, as disclosed in U.S. Pat. No. 4,625,376, the inside of the press shell is hermetically closed off from the outside so that there is no danger of contamination by penetration of a lubricant from the inside to the outside of the press shell. The press shell is connected in pressure-tight manner to supporting disks, which are mounted on a stationary support member. In this way it is possible to develop a certain excess pressure within the press shell by feeding in compressed air. This improves the quietness of travel of the press shell. Further quietness is obtained by connecting the two ends of the press shell precisely coaxially to each support disk by a centering device. This, together with the internal pressure produced by the compressed air, has the result that the press shell, upon rotation, slides only over the press shoe and is otherwise free of contact as it revolves around the support member. In other words, no guide elements, for instance ledges over which the press shell would slide upon rotation, are required within the press shell.
The forces occurring in the press nip of such a long-nip roll press are substantially greater than in a traditional two-roll press. The order of magnitude of the line force can be 1000 kN/m. The pressing forces which are applied to the machine frame by the support member and the mating roll are correspondingly high. It is therefore important that (as suggested by U.S. Pat. No. 4,643,802) a symmetrical arrangement of the support member, the press-shoe pressure chamber and the mating roll be provided. In this way, it is assured that the pressing forces are introduced into the machine frame at least predominantly in one and the same plane, namely in the press plane, the press plane being preferably (but not necessarily) substantially vertical. Oblique directions of force or directions of force which are staggered with respect to each other, which occur for instance in the apparatus disclosed in GDR Patent 79,919 and FRG Patent 3,030,233, are undesirable since they would result in disturbing transverse forces or bending moments.
The above-described guidance of the forces in the preferably vertical press plane may also be obtained when the center of the slide surface of the press shoe is displaced outside the press plane, as disclosed in U.S. Pat. No. 4,643,802. This displacement of the slide surface serves the purpose of obtaining an asymmetric pressing-pressure distribution in the direction of travel of the paper. More specifically, the pressing pressure is desired, commencing at the entrance into the press nip, to initially gradually increase up to a maximum, and then, shortly before the emergence from the press nip, to suddenly decline. In this way, remoistening of the web of paper by the felts, at the outlet from the press nip, is reduced.